With performance improvement of microprocessors, electronic devices are becoming smaller and lighter. Recently, the demand for ultra-thin mobile devices with maximized portability, such as smart phones and table PCs, is rapidly increasing. These electronic devices typically have a stacked structure with very limited room for a cooling system. For this reason, the need for an effective cooling system has arisen.
A heat pipe is a micro cooling system and is a representative power-free thermal control device. Conventional heat pipes consist of a vacuumed and sealed tube containing a predetermined amount of working fluid. The inner wall of the sealed tube is composed of a capillary structure having working fluid passages for moving a working fluid by capillary action. A heat generating means or a heating means is installed outside the sealed tube to be arranged near an end portion of the sealed tube, so that the end portion of the sealed tube functions as an evaporator (heating unit) that evaporates the working fluid. Meanwhile, a heat-dissipating means or a cooling means is installed outside the sealed tube to be arranged near a opposite end portion of the sealed tube, so that the opposite end portion of the sealed tube functions as a condenser (cooling unit) that condenses the working fluid.
This heat pipe transfers heat from a heat generating means and a heat-dissipating means to a heating means and a cooling means and vice versa with the use of latent heat, which is concomitant with frequent liquid-vapor phase changes caused by an evaporator and a condenser. Therefore, this heat pipe exhibits higher heat transfer performance (thermal conductivity) than a case of using a general pure metal. Accordingly, a heat pipe is widely used as a key component for heat transfer in various products including heat exchangers, cooling devices, and heat transfer devices.
However, a conventional heat pipe has a wick (capillary structure) that is typically made of a porous material, such as a metal net, sintered metal powder, or metal fiber, and which has a predetermined thickness or thicker, for circulation of a working fluid. Generally, the thinner the heat pipe, the poorer the performance of the heat pipe. Therefore, there is a limit in application of a conventional heat pipe to ultra-thin electronic devices.
In order to address problems associated with heat pipes employing a wick, recently, a pulsating heat pipe (PHP) using a thin tube with a small diameter as a closed tube has been developed. The pulsating heat pipe evaporates, transfers, condenses, and returns a working fluid through capillary action without using a wick.
However, as to conventional pulsating heat pipes, study and research have been mostly focused on the shape of a single-turn PHP and only liquid such as water has been used as a working fluid. Therefore, there still remains a problem that it is difficult to obtain satisfactory heat transfer performance required for a cooling device for use in electronic devices.
Therefore, there is an urgent need for development of an improved flat plate pulsating heat pipe that has high heat transfer efficiency and which can stably operate in ultra-thin electronic devices.
As a reference, conventional flat plate pulsating heat pipes are disclosed in the following patent documents.